A small drone as shown, does not have the distance capabilities to be launched from Ukraine.
It appears to be a false flag to justify a retaliation event on to Kiev.
I could be wrong, but I don't believe Putin would ever use the Kremlin as a target in a false flag attack. The embarrassment outweighs the propaganda value.
Also, why would you put 2 men on the ladder at the exact time?
Ukraine has used Mugin-5 and UJ-22 drones against oil storage facilities and other targets inside Russia many times. The Russians posted photographs of a crashed UJ-22 60km from Moscow earlier this year.
If I knew how tall the flagpole is, I could make some pretty good calculations.
This screen cap shows the drone just before it detonates. It approached from the left at maybe a 30-45 degree angle, and passed just behind the flagpole. It detonated in the frame after this one.
The wingspan at this angle is about half the height of the flagpole. If the flagpole is 25 feet tall, the apparent wingspan at this angle would be about right for a UJ-22, which has a 5 meter wingspan.
Range of UJ-22 is 800km. It's 650km from Kharkiv to Moscow.
Payload is 20kg, a small prox-fused warhead is definitely within Ukraine's capabilities.
I think it came from Ukraine. A symbolic strike, the target was the flagpole itself. Demonstrate the capability to strike the Kremlin directly, topple the flag if you get lucky, and show the Kremlin's air defenses are not really very effective.
And they have to deny it- the White House would never sign off on something this provocative.